all chemicals produced worldwide [14]. The value and role of catalysts in the industry cannot be overstated. The valorization of biomass is no exception [1–5].
As explained in Section 2.1.1., lignocellulose can be considered to be the main, or only, appropriate substrate for large‐scale, acid‐catalyzed processing into platform chemicals, and it accordingly holds significant industrial potential [4]. The net production of terrestrial plant cell walls alone has been estimated to be 150–170 billion tons per year [15]. There already exists a very significant industry revolving around lignocellulosic materials [16]. Of these, forestry is one of the largest with a mature global supply chain from sustainable forests through to saw mills, pulp and paper manufacturing, and distribution networks. This industry, along with paper and cardboard recycling, is ready and poised to supply large volumes of feedstocks to industries focused on the valorization of lignocellulose to produce platform and other chemicals. Logging slash (the side branches and other woody debris left during logging operations) would be an excellent source material for industrial processes, given the volumes of its production, the zero or negative cost associated with slash, and the existing environmental issues relating to slash [17]. Other potential high‐volume sources of materials for the sustainable acid‐catalyzed production of green chemicals, without disrupting any existing business or facilities, include waste streams of marine and fresh water algae aquaculture and nonedible and waste residues from agriculture, horticulture, and food production [4].
A goal of this chapter is to define efficient methods for the acid‐catalyzed conversion of biomass into targeted value‐added products. It analyzes current technologies, underpinning the relation of the method to the sustainability of the process. It also discusses the chemistry accompanying catalytic transformations of carbohydrates and lignin into specific products, defining the overall role of the acidic catalyst, solvent, and processing parameters. This is to strengthen the foundation for future sustainable developments of biorefineries.
2.1.1 Is an Acid the Best Catalyst?
Acid catalysts can be classified as Lewis acids or Brønsted acids and may be further classified as heterogeneous or homogeneous [18]. Brønsted acid‐catalyzed reactions are those in which molecules are activated by protonation of the substrate. A wide range of sites may be protonated, including carbonyl systems, alcohols, ethers, double bonds, etc., but not all protonation steps lead to chemical reactions. Often, the strength of the Brønsted acid, in combination with other reaction conditions, determine how and where a given substrate molecule reacts. This point will become clear as this chapter unfolds.
Conversely, Lewis acid‐catalyzed reactions are those in which the electron‐deficient center of a Lewis acid coordinates to a Lewis basic site on the substrate. The site and strength of the bonding may be predicated based on Pearson's HSAB (hard and soft [Lewis] acids and bases) principles, which predict that soft Lewis acids interact preferentially with soft Lewis bases and hard with hard [19]. When it comes to biomass, the majority of the active sites are hard Lewis bases, and so it is most common to encounter the application of hard Lewis acids to valorization chemistry.
In a third type of acid catalyst system, the mixtures of two different acid catalysts produce synergistic effects to deliver enhanced catalytic activity (Brønsted or Lewis) via an assisted acidity mechanism [20,21]. The assisted acid systems can be constructed from mixtures of two Brønsted acids (Brønsted acid‐assisted Brønsted acid catalysts), two Lewis acids (Lewis acid‐assisted Lewis acid catalysts), or Brønsted and Lewis acids (Brønsted acid‐assisted Lewis acid catalysts or Lewis acid‐assisted Brønsted acid catalysts) [20]. Under specific types of conditions, these mixtures can turn into superacidic systems [22], imparting very significant catalyst activity. A pictorial manifestation of the activation of the substrate by different types of acid catalysts is represented in Scheme 2.1, and throughout this chapter, we will make reference to the different types of acidity that are, or may be, at play.
As is evident from Scheme 2.2, acids can promote the valorization of all main classes of biomacromolecules. Only a few existing acid‐catalyzed processes can be considered to be sustainable, usually because of the nature of the substrates employed. Most methods that show promise in the laboratory are based on transformations of refined edible substances, such as low‐molecular‐weight saccharides and oligosaccharides, vegetable oils, or refined proteins [1,2,4,23]. The use of food products for the production of chemicals fails to meet sustainability requirements and must be avoided in large volume manufacturing, even though certain biorefinery processes have been readily engineered on a sizeable scale, such as production of lactates from sugar‐derived lactic acid (LacA), alkyl glucoside surfactants from mono‐ and oligosaccharides, or biodiesel fuel from edible seed oils [1,2]. In contrast, nonfood competitive materials, such as plant cell walls (lignocellulose), or oleaginous and keratinous waste are sustainable feedstocks for industrial chemical processing [4,24,25]. Examples of sustainable feedstocks include non‐food‐competitive waste products and side streams of forestry, horticulture and agriculture, vegetable oil refinery, animal husbandry, along with animal feather, wool, horns, and other indigestible fibrous proteins (keratinous feedstocks).
Scheme 2.1 Types of acid catalysts and acid‐catalyzed activations. Sub, substrate. H+, hydrogen ion; although Brønsted acids are conventionally denoted as H+, the hydrogen ion is known to form Lewis acid–Lewis base complexes, for instance, H13O6+ in aqueous solutions. M, electron‐deficient center of the Lewis acid catalyst. Source: Bodachivskyi et al. [4,7].